This invention relates to a semiconductor laser device and an optical pickup device, and in particular to an optical pickup device for use in an apparatus for optically recording or reproducing information on an information recording medium. This invention also relates to a semiconductor laser device and an optical pickup device that have a hologram device, and in particular to a semiconductor laser device and an optical pickup device for use in reading and writing a signal on an optical recording medium.
Conventionally there has been an optical pickup device as shown in FIG. 5. A laser chip 108 in a semiconductor laser device 101 of the optical pickup device emits a laser beam L which is incident on a diffraction grating 102. The laser beam L is split into a main beam L0 and two side beams L+1 and L−1 by the diffraction grating 102. The three beams (main beam L0 and side beams L+1 and L−1) are transmitted through a beam splitter 103 and transmitted through a collimator lens 104. The three beams are transformed into parallel beams by the collimator lens 104, and thereafter incident on an object lens 105. The object lens 105 focuses the beams on a surface of an optical disk 106 into a spot. Then, the three beams are reflected on the surface of the optical disk 106. The beams are transmitted successively through the object lens 105 and the collimator lens 104. The beam splitter 103 reflects the beams so that the beams are made incident on a photodetector 107. Consequently, the three beams are detected as information by the photodetector 107.
For another example, FIG. 11 shows a hologram-integrated type semiconductor laser device 180 of one red beam system. FIG. 12 shows an optical pickup device provided with the semiconductor laser device 180.
In this optical pickup device, light emitted from a semiconductor laser 157 is transmitted through a signal hologram 155. The light is diffracted into zero-order diffracted light, positive first-order diffracted light and negative first-order diffracted light by the signal hologram 155. Among these diffracted light rays, only the zero-order diffracted light is transmitted through the collimator lens 181 shown in FIG. 12 and transformed into parallel light by the collimator lens 181. The parallel light is transmitted through a quarter-wavelength plate 182. At this time, quarter-wavelength plate 182 generates phase difference of 45° between the light rays whose polarization directions are perpendicular to each other. Thereby, linearly polarized light is consequently transformed into circularly polarized light. Then the light is made incident on a raising mirror 183. The raising mirror 183 allows the light to be bent by an angle of 90 degrees and guided toward an optical disk 185. The light is concentrated on the optical disk 185 by an object lens 184.
Then, the light is reflected by the optical disk 185 to return the above-stated light path. That is, the light is transmitted through the object lens 184, bent by an angle of 90 degrees by the raising mirror 183 and made incident on the quarter-wavelength plate 182. The quarter-wavelength plate 182 further rotates the polarization direction of the light by an angle of 45 degrees, so that red light 186 generates whose polarization direction is rotated by an angle of 90 degrees in total through the forward path and the return path of the light. This return light 186 is transmitted through the collimator lens 181, and diffracted by the signal hologram 155 of a hologram device 154. The signal hologram 155 allows the positive first-order diffracted light 159 to be concentrated on the light-receiving device 158.
Other related arts of the present invention are disclosed in Japanese Patent Laid-Open Publication Nos. SHO 61-250844 and 2002-148436.
The optical pickup devices as shown in FIGS. 5 and 12 have such drawbacks as to degrade the signal detection characteristic and the servo characteristic of the optical pickup device because the beams reflected on the surface of the optical disk partially return to the semiconductor laser or its surroundings as optical noises. Also, these optical pickup devices has a drawback that the return light to the semiconductor laser generates the SCOOP (Self Couple Optical Pickup) phenomena exerting a harmful influence on the oscillation state of the original laser beam.
Specifically, in the optical pickup device shown in FIG. 5, parts Lm, Ls1, Ls2 of three beams reflected on the surface of the optical disk 106 return to the semiconductor laser device 101, as shown in FIG. 6. Consequently, the return beams Lm, Ls1 and Ls2 are reflected by the laser chip 108 and its surrounding part 111 in the semiconductor laser device 101, to goes back again the optical system located between the semiconductor laser device 101 and the optical disk 106. This causes the reflected return beams to be disadvantageously interfered with the original beam L as optical noises, which leads to significant deterioration of signal detection characteristic and the servo characteristic of the optical pickup device.
In the case of the SCOOP phenomenon, the gain of the laser beam L is increased by the reflected main beam Lm made incident on the light-emitting end surface 108a of the laser chip 108, so that the power of the laser beam L emitted from the laser chip 108 is undesirably increased. In addition, since the reflected main beam Lm fluctuates according to the surface state of the optical disk 6, the power of the laser beam L also disadvantageously fluctuates in accordance with the above-mentioned fluctuation.
As a method for eliminating the optical noises, the Japanese Patent No. 2565185 discloses that a slant surface is provided on a beam incident part of a header of the semiconductor laser device, or that a reflectionless coating is provided on the beam-emitting end surface.
Also, as shown in FIG. 7, a slant surface 212 is provided on an end surface 211a to reflect a side beam Ls2 in the direction of arrow R, so that the reflected side beam Ls2 is restrained from returning again to the optical system located between the semiconductor laser device 101 and the optical disk 106.
Further, as shown in FIG. 8, a reflector 113 is attached to the end surface 111a of a header section 111 of a stem 110 so as to reflect a beam Ls2 in the direction of arrow R by the reflector 113.
However, all of the above-stated methods have difficulties in formation of the reflectors and so on, which causes low productivity of the semiconductor laser devices.
Specifically, the header section 111 is formed integrally with the stem 110 by press molding in general. That is, the header section 111 is formed by pressing a hoop-shaped (elongated thin plate-shaped) iron material 100 shown in FIG. 9A with a metal mold 701. Since a protruding portion, which becomes the header section 111, is formed on the originally flat plate iron material 100, it is required to apply a very strong pressure to the metal mold 701. Particularly, in order to flatten the end surface 111a of the header section 111 as shown in FIG. 9B, the greatest force is to be applied to a portion 702 opposed to the end surface 111a in the metal mold 701. In this case, to form the slant surface 212 (see FIG. 7) on the end surface 111a of the header section 111, it is also required to provide the metal mold 701 with a slant surface forming portion constituted of a projection or a recess corresponding to the configuration of the slant surface 212. However, the slant surface forming portion, which is provided at the portion 702 to which the maximum pressure is applied as described above, is disadvantageously easily collapsed by pressure. Therefore, in the case where the slant surface 212 is provided on the portion 111a, extremely poor productivity results. Then, this sometimes leads to the destruction of the metal mold 701 itself in the worst case. Accordingly, there is a problem that it is realistically impossible to carry out mass production of the stem 210 that has the header section 211 on which the aforementioned slant surface 212 is directly formed.
Moreover, the method for attaching the reflector 113 to the end surface 111a of the header section 111 has the following problem.
The reflector 113 is required to be extremely accurately attached to the end surface 111a of the header section 111. Depending on the design of the optical system of the optical pickup device, it can also be considered that the reflected side beam Ls2 is incident on the neighborhood of the boundary between the laser chip 108 and the header section 111. Taking the incidence of the reflected side beam Ls2 into consideration, the end surface 111a in the neighborhood of the boundary is required to be covered with the reflector 113. However, it is extremely difficult to attach the reflector 113 so as not to bring the reflector 113 in contact with the laser chip 108. Moreover, when the reflector 113 protrudes from the end of the header section 111, the laser beam L of the laser chip 108 is partially blocked off by the reflector 113 as shown in FIG. 10, and this leads to a problem that the light emission characteristic of the laser chip 108 is degraded. That is, by attaching the reflector 113 to the end surface 111a, a far-field pattern (in the portion of the dotted line D1), which should be essentially present, disadvantageously disappears.
On the other hand, as a counter-measure against the above-stated SCOOP phenomena, there is a method for increasing reflectance of the light-emitting end surface 108a of the laser chip 108 so as to reduce the quantity of return of the reflected main beam Lm to the inside of the laser chip 108. However, the increase in the reflectance of the light-emitting end surface 108a disadvantageously leads to reduction of a differential efficiency ηd of the laser beam L of the laser chip 108, which reduction is unsuitable for increase of output power or reduction of current in the laser chip 108.
Moreover, in the semiconductor laser device and the optical pickup device shown in FIGS. 11 and 12, the signal hologram 155 generates the zero-order diffracted light and the negative first-order diffracted light together with the positive first-order diffracted light 159. The zero-order diffracted light is incident on the light-emitting point of the semiconductor laser 157 as a return optical noise. Then, the zero-order diffracted light exerts such a harmful influence on the semiconductor laser 157 as to provide the laser beam with unstable oscillation.
Also, the negative first-order diffracted light passed through the signal hologram 155 is reflected by a top surface of a stem 152 and inner surfaces of a cap 153, which light may be made incident as unnecessary stray light rays on the light-receiving device 158, consequently causing an increase in signal offset and degrading the signal processing capability of the optical pickup device.